Material transfer apparatus are commonly employed for transferring particulate or fungible materials from one container to another, or from a supply container to a desired location. In abrasive blasting operations, for example, a hopper is commonly employed for containing a supply of sand or other abrasive material, and an evacuated conduit or hose may be connected between the hopper and the grit blasting nozzle, the supply hose necessarily being of sufficient length to reach from the hopper to the work area. In the use of such systems for sand blasting operations and the like, a high pressure air supply connected to the abrasive ejector nozzle may be employed to effect a vacuum in the supply hose sufficient to provide a flow of abrasive to the nozzle from a hopper remote from the work station. It will be understood by those in the art that, while the flow of abrasive material through the nozzle in a typical system may vary and may occasionally be interrupted because of obstructions in the hose or nozzle, flexing of the hose, or the like, such flow stoppages are not of major concern in typical sandblasting operations in that they may be readily corrected. Variations in flow rate may be compensated for by corrective procedures employed by a skilled operator as he observes and controls the operation of the blasting nozzle upon a workpiece.
Similar transfer systems have been employed for feeding particulate abrasive materials to modern fluid powered abrasive cutting systems such as those employed abrasive water jet cutting nozzles. Such systems employ an ejector nozzle powered by a liquid, such as water, supplied at very high pressures (e.g., in the order of 50,000 p.s.i. or greater). In typical applications, the high pressure fluid flow through the nozzle induces a vacuum in a supply tube communicating with a hopper containing an abrasive grit, such as garnet, silica, aluminum oxide, or the like. Air flow is induced within the supply tube and the abrasive is thus drawn from the hopper to the cutting nozzle. The vacuum produced by the flowing liquid is not as great, however, as that produced by a typical air powered sandblasting nozzle or the like, and the practicable length of tubing extending to the hopper containing the abrasive is thus undesirably limited. In automated, robotically controlled fluid jet cutting systems, for example it may be desired that the robot and the cutting jet nozzle be moveable over substantial distances and freed from constraints resulting from limited range between the abrasive hopper and the water jet nozzle. In our experiments, we have found that the supplemental use of air pressure to urge the abrasive grit toward the cutting nozzle tends to induce problems, such as stoppages and erratic flow. Thus, the limited range of the abrasive supply hose is an undesirable limitation in current automated robotic cutting systems.
Of perhaps even greater significance, however, is the requirement in such automated systems for a continuous and steady flow of the abrasive to the nozzle for effecting a continuous, even cut or Kerf through the workpiece. This requirement is of major importance when automated cutting operations are entailed and when such abrasive cutting jets are employed for cutting various composites, such as composite laminates of graphite epoxy, or Kevlar. In the latter instances it is essential that the abrasive be continuously entrained in the fluid jet to preserve the structural integrity of the workpiece. This is because the high pressure liquid does not effectively penetrate such materials without the additional, abrasive action of the grit, for the following reasons.
In normal, continuous cutting operations, the ejected water and abrasive jet penetrates the workpiece and is collected in a suitable receptacle or "catcher". Thus, kinetic energy of the liquid jet, which may exit the nozzle at velocities in excess of twice the speed of sound, is dissipated in the catcher. In the event however that the abrasive flow is interrupted and the liquid jet does not penetrate the workpiece, the jet becomes trapped within the body of the laminate rather than passing through, and the energy of the jet becomes dissipated within the laminate. The end result of such a stoppage is that the liquid stream is diverted outwardly within the laminate and the very high inertial and kinetic energy entailed in the liquid stream may produce delamination, and effect destruction of the workpiece. As will be understood by those in the art, large composite workpieces entailed in applications such as aerospace manufacturing may be the product of several previous manufacturing and assembly operations and may thus represent substantial manufacturing costs. It is thus apparent that interruptions of the supply of grit to such a liquid abrasive jet nozzle may entail serious consequences and should be avoided.
It has been found that gravity feed supply lines provide a more reliable and constant flow of abrasive to such water jet cutters. However, conventional gravity flow systems would not be practicable for automated, robotically operated systems because the provision of a large supply hopper adjacent to the cutter and the work area would be impracticable, and the use of a smaller, more portable hopper entails the disadvantage that the hopper must be refilled at undesirable frequent intervals.